Alloy steel and method



tas

This application for patent is a continuation-in-part of my copending application Ser. No. 7,554, filed February 9, 1960, and entitled Alloy Steel and Method, now abandoned, and a companion of my copending U. S. application, Ser. No. 39,221, filed of even date herewith. The invention relates generally to the chromium-nickel grades of alloy steels. More particularly, it concerns a heathardenable chromium-nickel alloy steel as well as a method for conditioning such a steel for hardening, and to the products fashioned therefrom, both in the prior-to-hardening condition, and in precipitation-hardened form. In some respects the steel of the present invention may be viewed as an improvement over the steel of my companion application for patent.

One of the objects of my invention is to provide a chromium-nickel alloy steel, as Well as various forms, shapes and fabricated products produced therefrom which are capable of being hardened from an annealed condition with a single-step, low-temperature ageing treatment, or, as desired, with a double-step treatment, to achieve high strength under sustained operating conditions of either low or high temperatures, or bot Another object is to provide a highly eifective method for conditioningalloy steels of the general character described, wherein, following shaping, forming or other fabrication of the metal in annealed condition and while in soft, formable and machinable state, the metal thereafter,

and in one-step operation, or with a double treatment, is

quickly, economically and reliably hardened and strengthened by way of a simple and comparatively low-temperature ageing treatment of relatively short duration.

Other objects and advantages will be obvious in part and in part more fully pointed out during the course of the following disclosure.

Considered in the light of the foregoing, my invention resides in the novel combination of component elements, in the compositions of materials thus obtained, in the prevailing conditions of treatment, in the several operational steps, and in the relation of each of the same to one or more of the others as described herein, the scope of the application of which is indicated not only in the following description but in the accompanying claims.

As a preliminary to the detailed disclosure of my invention, it may be noted that the invention is directed primarily to an improvement in the well-known chromium-nickel alloy steels. Of this group of steels the austenitic chromium-nickel grades are defined as having a chromium content ranging from about 10% up to about 35%. The nickel content may range from something above on up to 25% or more. Nickel is included for the dual purpose of improving the stainless qualities of the metal and to control the structure of the steel and with this the softness, ductility, and magnetic qualities thereof. With nickel high the resulting product is essentially austenitic. Conversely, with nickel low relative to chromium, the product is either typically martensitic or ferritic.

Where desired, in the known chromium-nickel steels various alloying metals may be included in small amounts to impart special qualities. Typical of such additions are aluminum, copper, molybdenum, manganese, cobalt, silicon, sulphur, phosphorus, tungsten, vanadium, titanium, columbium and the like. Aside from the chromiumnickel content together with one or more of the alloying 3,083,095 Patented Mar. 26, 1963 riveting, or like joining treatment, nevertheless such steels,

with their high values of both chromium and nickel, are virtually immune to phase-transformation when subjected to heating and subsequent quenching. The soft austenitic condition, characteristic of these steels, is retained throughout heat-treatment; the steels are not hardenable by heattreating methods to improve their strength and hardness. in some respects this is viewed as a disadvantage.

Another disadvantage of the austenitic chromiumnickel stainless steels is that upon rolling or drawing, or Where the work process conducted thereupon is primarily in a single direction, the resulting mechanical properties vary relative to the direction of working; the yield strength measured transverse to the working direction is not developed to the same extent as in the direction of working. This difference is particularly noted in compression tests. And in many applications such a difference may not be tolerated.

The tendency of the austenitic steels to harden upon working requires interruption of the forming operation and the strains induced by working, then relieved by subjecting the metal to intermediate anneal. Frequently, such intermediate anneal is attended by a loss of the nice dimensionality which has already been imparted to the product, as through light scaling or perhapsdistortion.

Contrasted with the austenitic chromium-nickel stainless steels are the so-called precipitation-hardenable stainless steels, that is, those steels which are hardenable by heat-treatment. Typical of. the precipitation-hardenable steels are the Armco 17-7PH, 17-4PH and PHl5-7Mo grades (these respectively analyze about 17% chromium, 7% nickel, 1% aluminum, and remainder iron, for the 17-7PH; about 17% chromium, 4% nickel, 4% copper, and remainder iron, for the 17-4PH; and about 15% chromium, 7% nickel, 2% molybdenum, 1% aluminum, and remainder iron, for the PI-I157Mo grade).

In their hardened condition, that is precipitation-hardened, these steels display many highly desirable mechanical properties. Illustratively, the steels are of high tensile strength. And in their annealed condition the steels possess good working and forming characteristics.

A disadvantage of many known precipitation-hardenable chromium-nickel stainless steels, however, is that while the prior art precipitation-hardened products possess admirable strength when subjected to service at room temperatures, these qualities unfortunately are lost once the products are subjected to use at elevated temperatures, say above about 900 An object of my invention, therefore, is to provide a steel of the chromium-nickel type, which steel While in the annealed condition readily lends itself to fabrication Referring now to the practice of my invention, I find I that a proper combination of the ingredients copper and I molybdenum with chromium and nickel in a steel of low carbon content achieves an alloy steel which is subsequently hardenable by heat-treatment from a soluble solution-treated (annealed) condition. This hardening is had by way of a singlerstep hardening operation of comparatively short duration conducted at relatively low temperatures', say from about 850 F. up to about 1350" F. Where desired, as for example for maximum for-mability in the solution-treated condition, hardening is had by a two-step treatment. In both instances the resulting steel is noteworthy in that it requires a total alloy content of minor amount. The hardened steel is strong and hard both at room temperatures and at high temperatures. The mechanical properties of my steel stand out boldly and in favorable comparison with known and available precipitation-hardened steels.

Broadly, the alloy steel of my invention essentially consists of about 3.0% to 14.0% chromium, about 2.5% to 10.0% nickel, molybdenum about 4.0% to 12.0%, copper about 1.0% to 5.0%, with remainder substantially all iron, all percentage figures being by weight. Carbon is included in amounts of about 0.01% to 0.15%. Manganese also may be present in the amount of about 0.05% to 2.50%. Phosphorus is present in amounts up to 0.05% maximum, sulphur up to 0.05% maximum, and silicon from about 0.05 to 2.50%. Where desired, columbium is employed in amounts up to about 1.0%. At times, and for various specific purposes, vanadium may be included. in my steel up to about 1%, titanium and/ or zirconium up to about 0.50%, boron up to about 0.005%, and nitrogen up to about 0.12%. In some instances, where extreme hardness is desired and some calculated sacrifice in ductility of resulting steel is permissible, tungsten may be substituted for molybdenum, up to amounts of about 7% tungsten.

The percentage of molybdenum employed in my steel and its relation to the chromium content thereof are both critical; with a molybdenum content below 4% the desired precipitation-hardening effect is not had. If the steel is either too low in chromium or too high in molybdenum, practical difficulties are encountered I find that with a chromium content near the lower part of the given range molybdenum may be somewhat higher than would otherwise be permissible; while if the chromium content approaches the upper limit of permissible range, then the ducted at comparatively low temperatures and enduring for relatively short intervals of time as more particularly described hereinafter. And the hardened products thus produced thereby acquire enhanced qualities of hardness and strength, qualities which are incident to the hardening treatment. And noteworthy is the fact that these qualities are retained during prolonged operation at both room temperatures and elevated temperature.

For superior results in a single-treatment steel within the foregoing broad range, I prefer a more limited range of chromium-nickel-molybdenum alloy steel, this analyzing about 3.0% to 14.0% chromium, nickel about 2.5% to 6.5%, molybednum about 4.0% to 12%, copper about 1.0% to 5.0%, carbon up to about 0.12% maximum; manganese. up to about 3.0% maximum; phosphorus up to about"0.05% maximum; sulphur up to about 0.05% maximum; silicon about 0.05% to 2.0%; columbium up to about 0.75%; remainder substantially all iron.

The double-treatment steel, also within the broad composition range set forth above, preferably analyzes about 3.0% to 14.0% chromium, 3.5% to 10% nickel, 4% to 12% molybdenum, 1% to 5% copper, up to 0.12% carbon, up to about 3.0% manganese, up to about 0.05 each of sulphur and phosphorus, up to 2.0% silicon, up to about 0.75% columbium, and remainder substantially all iron.

I preliminarily heat the single-treatment steel of my invention, to bring it into a soft, ductile condition which is suitable for fabricating operations, by subjecting the metal to a solution-treatment at temperature ranging from about 1600 F. up to as high as about 2000 F. I

find that these relatively high annealing temperatures effectively serve toplace the metal in an austenitic condition where the molybdenum constituents are in solution. I further find that this single-treatment steel when cooled as by quenching in air, oil or water, is transformed to a substantially martensitic condition, with molybdenum constituents retained in solution. Although the duration of the annealing treatment is not too critical I find satisfactory, and accordingly prefer, a treatment enduring for approximately one-half an hour. The annealing treatment is conducted in any heat-treating furnace suitable for the purpose. The metal is brought to temperature and held at prevailing temperature for a time sufficient to place the molybdenum constituents in solution. With molybdenum constituents in solution I quench the metal, preferably in oil, to about room temperature. This treatment usually is effected at the steel mill.

In its annealed'o'r solution-treated condition, my singletreatment steel u-pon quenching displays a. structure which is basically martensitic. This is clearly discernible under the microscope. Moreover, it is usually free of deltaferrite. It is ductile and possesses good qualtities of directionality, together with hardness of about Rockwell C30.

The double-treatment steel of my invention preferably is solution-treated at a temperature of some 1850 F. to 2150 F. for one-half hour or moreand then cooled to room temperature. The steel is largely in the austenitic condition with minor amounts of delta ferrite. The hardness in this condition is something under Rockwell B100. This treatment, where employed, likewise is usually done at the mill.

The steel of my invention following solution-anneal is ordinarily shipped to a customer fabricator. It is then readily formed and fabricated as by punching, bending, stretching, shrinking, or the like, or by drilling, cutting, threading, and so forth. It is successfully and readily brazed or soldered. Or it canbe welded in accordance with the practice of many well-known welding operations. In short, this steel in its soft and ductile pre-hardened condition can be readily processed as desired; especially is this true of the double-treatment steel as annealed at temperatures significantly higher than those used for the single-treatment steel.

Upon conclusion of the fabrication operations of the single-treatment alloy I harden the same, as by a single-.

stage ageing treatment conducted at relatively low temperature for but a short period of time. To this end, I subject the steel and products formed therefrom to an ageing temperature in range from approximately 850 F. up to as high as about 1350 F. for a period of approximately one hour. Following the ageing treatment, I cool the steel, as in air or water. This single-step, low-temperature ageing treatment, of relatively short duration, brings about a precipitation of a molybdenumrich phase. It may be supposed that it is this precipitated phase, rich in molybdenum and distributed uniformly throughout the steel to an extent such as to be seen with the microscope, to which may be attributed the hardness and high physical strength realized in the hardened steel.

The short duration of the ageing treatment and the comparatively low temperatures at which ageing is conducted combine to ensure a minimum of scaling while preventing undesired distortion of the product undergoing treatment.

For the double-treatment steel of my invention (preferably solution-treated at 1700 F. to 2150 F. at the mill), I employ a double heat-treatment for hardening the metal. In this the steel in solution-treated condition, and in the form of the desired article, is first brought to a temperature of about 1100 F. to 1800" F. for a time up to 5 hours or longer and then cooled at a temperature of 150 F. to 300 F. for a time up to hours or longer. This elfects transformation to a martensitic condition.

Transformation also may be had by mechanically coldworking the solution-treated metal, as in forming and fabrication, until a ferritic condition is developed. In general, however, I employ the transformation heat-treatment.

Hardening of the transformed steel now is had by reheating at a temperature of 800 F. to 1250 F. for a period of time ranging up to 12 hours or longer, and quenching. The hardness realized ordinarily comes to Rockwell C38 to C55.

As suggested, following quenching, the steel whether hardened by single-treatment or by double-treatment, is found to have acquired high hardness and high strength. Importantly, and unlike many known precipitatiomhardened alloys, the metal retains these properties even following prolonged operation at high operating temperatures. This I attribute to the stability of a molybdenum-rich phase present in the steel in the hardened condition. The hardness and strength of my steel at room temperature is substantially retained in prolonged high-temperature service. This important advantage may be noted upon comparison with many alloys, which in sharp contrast characteristically suffer a loss in these qualities, when operating at elevated temperatures.

As more specifically illustrative of the practice of my invention, I prefer a steel analyzing: approximately 10% to 12% chromium, 5% to 6% nickel, 5% to 6% molybdenum, 3.5% copper, and remainder substantially all iron; more particularly one analyzing about 11% chromium, 5% nickel, 6% molybdenum, 3.5 copper and remainder substantially all iron. The carbon content is low, usually lers than 0.12%. This steel can be hardened both by single-treatment and by double-treatment methods.

A preferred composition of a single-treatment steel analyzes approximately 12.5% chromium, 5.0% nickel, 5.0% molybdenum, 3.5% copper, and remainder substantially all iron. The carbon content is low, .05% max. Another single-treatment steel, while lower in chromium content, is comparatively rich in nickel and molybdenum, this analyzing: approximately 4.0% chromium, 4.0% nickel, 5.0% molybdenum, 3.5% copper, and remainder substantially all iron. A further preferred single-treatment steel analyzes about 8.0% chromium, 4.0% nickel, 7.0% molybdenum, 3.5 copper, and remainder substantially all iron.

Specific steels of the double-treatment type analyze: about 9% chromium, 8% nickel, 6% molybdenum, 3.5 copper, and remainder iron for one; about 5% chromium, 10% nickel, 6% molybdenum, 3.0% copper, and remainder iron for another; about 11% chromium, 7% to 8% nickel, 5% to 6% molybdenum, 3% copper, and remainder iron for a third. Of these preferred steels, excellent weldability is had for the low chromium alloy example. For good corrosion resistance I prefer the steels analyzing: about 12% chromium, 6% to 7% nickel, 5% molybdenum, 3% copper, and remainder iron for one; and about 13% chromium, 5% to 6% nickel, 5% molybdenum, 3% copper, and remainder iron for another.

The alloy steels of my invention with less than about 8% chromium are not considered to be stainless steels because I find that, where the chromium content is less than about 8%, there is a loss of passivity.

6 Within the broad range of composition of my steel, I prepared a number of steels of single-treatment steels of composition given in Table I below:

Table I SPECIFIC EXAMPLES OF SINGLE-TREATMENT Cr-Ni-Mo-Ou STEELS Heat 0 Mn P S Si Cr Ni Mo Cu Ch O38160 047 .39 .011 .016 .41 12.44 5. 40 5. 24 3. 50 .19 O38157 040 .39 .010 .018 .51 9. 83 5. 88 6.07 3. 47 .17 R861 029 .59 .009 .011 .59 10.51 4. 21 5. 82 3. 60 .20 R862 041 .64 .009 .010 .53 9. 30 4.10 6. 3. 52 25 R214.. 031 .41 .010 .014 .48 9. 26 4.10 6. 89 3. 64 .21 R2143 029 .44 010 .012 .42 8. 42 4.08 8. 03 3. 59 .22 R2144 029 .38 009 .014 .44 7. 75 4. 10 8.83 3. 64 25 Specimens from the several steels identified in Table I- were solution-treated, then precipitation-hardened by heating at 1050 F. and cooling, in accordance with the practice of my invention. The mechanical properties of these specimens were determined at room temperature by known and accepted testing practices. The properties of the specimens in the hardened condition and in the annealed condition as well, for two of the specimens (heats 038160 and R861) are given in Table 11 below:

Table II ROOM: TEMPERATURE IVIECHANICAL PROPERTIES OF THE STEELS OF TABLE I No'rn.(a) Annealed at 1,900 F. for hr. and oil-quenched. (h) Annealed as in (a) and hardened at 1,050 F. for 1 hour and air-cooled.

From the results given in Table II (notably the results for heats 038160 and R861 in the annealed and in the hardened condition) an important increase is observed in the mechanical strengths of these test specimens as a result of the hardening treatment. Moreover, there is noted an increase in strength ((illustratively heats R2143 and R2144) where the percentage of molybdenum is relatively high.

A number of double-treatment steels of composition well within the broad range of composition set forth above also were prepared, the chemical analyses of those being given in Table III below:

Table III Heat No. C Cr Ni 1 Mo Cu Cb The steels of Table III were annealed at 2000 F. for /2 hour and water-quenched. They were then reheated at 1400 F. for 1% hours and cooled to room temperature in order to transform the metal to martensite. Following transformation they were reheated at 1050 F. for 1% hours and air-cooled. The room temperature mechanical aosspes 7 properties of those steels in the final hardened condition are given in Table IV(a) below:

1 Table IV(a) ROOM TEMPERATURE IVIECHANIGAL PROPERTIES OF THE STEELS F TABLE III AFTER TRANSFORh IATION AND HARDENING Heat N0. Ult. ten. 0.2% y s Percent Percent Rockwell str., p.s.i. p.s.i. e1. in 2 red. area hardness 226, 000 175,000 8 13 C48 210, 000 178, 000 15 32 C46 203, 000 151,000 15 28 C46 228. 000 187, 000 7 7 C49 216, 000 169, 000 12 23 C47 216, 000 156, 000 12 17 C46 181, 000 106, 000 19 31 C42 220, 000 186, 000 11 26 048 211,000 180, 000 12 31 C47 218, 000 192, 000 13 35 C48 In the annealed condition the hardness mounted to Rockwell B82 to 99, it being about Rockwell B87 for most of the examples.

I find that even better mechanical properties are had where the solution-treated steel is given a conditioning treatment and a refrigerating treatment for transformation and then the hardening treatment. More particularly, the steels of Table III were annealed at 2000 F. for ,42 hour and water-quenched, following which they were reheated at 1400 P. for 1 /2 hours and cooled to room temperature, then refrigerated at -50 F. for 2 hours to effect complete transformation. Following this they were hardened by reheatingat 1050 F. for them and air cooling. The mechanical properties for the finally hardened steelsare given in Table 1V(b):

Table IV(b) ROOM TEMPERATURE MECHANICAL PROPERTIES OF THE STEELS OF TABLE III AFTER CONDITIONING AT 1400 F. AND TRANSFORMING AT -50 F., THEN HARDEN- Heat N0. Ult. ten. 02% y s Percent Percent str., p.s.i. p.s.i. el. in 2 red. area Table V STRESS-RUPTURE STRENGTH OF TWO OF THE STEELS 013 TABLE I V Load for fracture (p.s.i.) Heat No. Test temp 7 100 hrs. 1,000 hrs.

1 Annealed or solution-treated at 2,000 It, my, lm-oil quench, followed by reheating at 1,100 F. for 1 hr. and air cool.

Consideration of the results reported in Table V shows that at a prevailing temperature of approximately 1000 F. the specimen of lower molybdenum content (heat R861, with 5.8% molybdenum) had hour life under a load of"95,000-p.s.i. The specimen of higher molybdenum content (heat R862, with molybdenum content of 6.75%) revealed a 100 hour life at ,000 p.s.i. The 1000 hour life of the two samples at 10 F. was, respectively, 85,000 p.s.i. and'87,000 p.s.i.

At a more elevated temperature, namely 1100" F., specimens from the same two heats (R861 and R862) had 100 hour lives of 163,000 p.s.i. and 65,000 p.s.i., respectively. The 1000 hour life of the two, respectively, amounted to 50,000 p.s.i. and 51,000 p.s.i.

In addition to excellent mechanical properties at room temperatures and at elevated temperatures it is to be noted that the addition of important percentages of molybdenum ensures that the steel possesses requisite values of mechanical strength, this without appreciably impairing ductility of the metal While in the solution-treated condition.

A comparison of the strength or" my new preferred steel (conveniently referred to as 11Cr, SNi, 6M0) with accepted steels generally intended for somewhat similar service is given in Table VI below:

Table VI COMIPARISON OF PROPERTIES OF 17-4PH, FEM-4M0 AND llCr-SNi-SMO STEELS Study of the results of Table VI reveals that the known PHl4-4Mo steel, wherein approximately 1.0% to 3.5% molybdenum has been substituted for a generally like quantity of chromium of the conventional 17-4PH steel, has about the same room temperature strength as the l7-4PH steel, this regardless of the substantial molybdenum addition (1.0% to 3.5%). With greatly increased molybdenum, however, and a proper proportioning of chromium and nickel contents, as in my steel, an important increase in room temperature strength is achieved. Even more important, however, with the high molyh denum steel of my invention there is had great high temperature proper-ties, both for the 100 hour load as well as the 1000 hour loading.

Illustratively my 11Cr-5Ni-6Mo steel has a tensile strength at room temperature in excess of 240,000 p.s.i., while at 1000 F., the specimens sustain a load in excess of 100,000 p.s.i. for 100 hours, and fracture only at loads in excess of 85,000 p.s.i. upon 1000 hours testing. This contrasts well with the 50,000 to 70,000 p.s.i. 100 hour life and 25,000 to 50,000 life at 1000 hours for the known 17-:PH and PHl4-4Mo steels.

As a further feature, the stainless steels of my invention as contrasted with the available 17-4PI-I and PH14- 4M0 steels, achieve great hardness when aged at temperatures substantially in excess of those customarily employed. In point of fact, While the 17-4PH and PH14-4Mo steels suffer a decrease in the maximum obtainable hardness as the temperature of heat-treatment (ageing) rises above about 900 R, my steel achieves an increase hardness where the ageing temperatures are 1000 F. and even 1100 F. The maximum obtainable hardness falls 011 only where the ageing temperature substantially exceeds 1100 F., say about 1200 F. But even at 1200 F., however, the hardness had does not fall below that obtained with the 900 F. heat-treatment. This result is in sharp contrast to that realized in the 174PH and PH144M0 steels where, as noted above,

Table VII ROGKYVELL HARDNESS VS. HEAT-TREATMENT TEMPERATURES FOR THREE GRADES OF STEEL Solution- Age-hardened (1 hr. heating) Grade treated.

900 F. 1,000 F. 1,100 F. 1.200" F.

l7-1PH C31 C45 C43 C37 C30 FEM-4M0 C31 C45 C43 C38 C30 llCr-5Ni-6Mo O31 C45 C46 G52 045 With a preliminary solution-anneal at about 1900 F. to 2000 F., all three steels (17-4PH, PH14-4M0 and llCr-SNi-6Mo) acquire a hardness of about Rockwell C31. Similarly, when the steels are subsequently agehardened for one hour treatment at temperature of approximately 900" F. they all display the same hardness, namely about Rockwell C45 At an ageing temperature of 1000 F., however, the 17-4PH and PH14-4Mo steels 'both begin to sufier a decrease in the obtainable hardness, down to value of C43, this is in sharp contrast with the hardness had in my steel where, it is noted, there is achieved an increase in hardness to Rockwell C46. And this distinction becomes even more pronounced with ageing at the higher temperature of 1100 F. For while the maximum hardness values of the 17-4PH and 14-4Mo steels fall to values of Rockwell C37 and C38, quite on the contrary the maximum hardness of my steel is found actually to increase up to a value of Rockwell C52. With the ageing temperature naised to about 1200 F., the specimens of 17-4PH and PH14-4Mo continue their drooping characteristic, and fall off to a hardness as low as Rockwell C30. The specimens of my llCr-SNi- 6M0 steel, however, aged as disclosed herein, achieve a decreased hardness of Rockwell C45.

Thus it will be seen that I provide in my invention a steel land method of heat-treating the same, in which the many objects hereinbefore stated are successfully achieved. My new steel displays markedly superior mechanical properties, particularly strength and hardness, and is well suited to duty at room temperatures, as well as at temperatures ranging up to about 1100 F. Advantageously, the fall-off in strength is comparatively 'gentle, even upon further increase in the temperature of use.

Moreover, in my steel it is to be noted that, upon agehardening, the steel achieves considerable resistance to wear, abrasion and galling. And, also as noted, in my steel, upon hardening, there is had great strength not only at prevailing room temperatures but at high operating temperatures, as well.

The steels of my invention lend themselves admirably not only to the production of castings but also to the production of wrought articles, and these of intricate detail and form, all with close response to nice tolerances. As well the metal in the solution-treated or annealed condition can be machined, welded and formed with admirable facility.

It becomes apparent from the foregoing that I provide in my invention a chromium-nickel-molybdenum alloy steel, together with a single-treatment method of hardening such steel through a low-temperature-ageing process of short duration, as well as a double-treatment, in the course of both of which I achieve the various objects pointed out hereinbefore, and these along with many thoroughly practical advantages.

Obviously, many embodiments may be made of my invention as disclosed herein. As well, many changes and modifications will suggest themselves to those skilled in the art, as concerns the disclosed embodiments. Accordingly, I desire the foregoing disclosure to be considered as simply illustrative, and not as a limitation.

I claim as my invention:

1. A single-treatment precipitation-hardenable chromium-nickel-molybdenum alloy steel essentially consisting of about 3.0% to 14.0% chromium, 2.5% to 6.5% nickel, 1.0% to 5.0% copper, 4.0% to 12.0% molybdenum, up to .75% columbium, and the remainder essentially iron.

2. A precipitation-hardenable stainless steel essentially consisting of approximately: 10% to 12% chromium, 5% to 6% nickel, 5% to 6% molybdenum, 1.0% to 3.5% copper, and remainder essentially iron.

3. A precipitation-hardenable stainless steel essentially consisting of approximately: 8.0% chromium, 4.0% nickel, 7.0% molybdenum, 3.5 copper, and the remainder essentially iron.

4. A precipitation-hardenable stainless steel essentially consisting of approximately: chromium 10.0%, nickel 6.0%, molybdenum 6.0%, copper 3.5%, and remainder essentially iron.

5. A precipitation-hardenable stainless steel essentially consisting of approximately: chromium 4%, nickel 4%, molybdenum 5%, copper about 3.5%, and the remainder essentially iron.

6. A precipitation-hardenable stainless steel essentially consisting of approximately: chromium 12.5%, nickel 5.0%, molybdenum 5.0%, copper 3.5%, and the remainder essentially iron.

7. A double-treatment precipitation-hardenable stainless steel essentially consisting of about 9% chromium, 8% nickel, 6% molybdenum, 3.5% copper, and remainder essentially iron.

8. A double-treatment precipitation-hardenable stainless steel essentially consisting of about 5% chromium, 10% nickel, 6% molybdenum, 3% copper, and remainder essentially iron.

9. A double-treatment precipitation-hardenable stainless steel essentially consisting of about 11% chromium, 7% to 8% nickel, 5% to 6% molybdenum, 3% copper, and remainder essentially iron.

10. A double-treatment precipitation-hardenable stainless steel essentially consisting of about 12% chromium, 6% to 7% nickel, 5% molybdenum, 3% copper, and remainder essentially iron.

11. A double-treatment precipitation-hardenable stainless steel essentially consisting of 13% chromium, 5% to 6% nickel, 5% molybdenum, 3% copper, and remainder essentially iron.

12. Precipitation-hardened wrought or cast alloy steel articles essentially consisting of approximately: carbon .05% max., chromium 10% to 12%, nickel 5% to 6%, molybdenum 5% to 6%, copper 1% to 3.5%, and remainder essentially iron.

13. Precipitation-hardened wrought or cast alloy steel articles essentially consisting of approximately: carbon .05% max., chromium 11%, nickel 7% to 8%, molybdenum 5% to 6%, copper 3%, and remainder essentially iron.

References Cited in the file of this patent UNITED STATES PATENTS 1,863,159 Hadfield June 14, 1932 2,528,638 Clarke Nov. 7, 1950 2,532,117 Newell Nov. 28, 1950 2,687,955 Bloom Aug. 31, 1954 2,797,993 Tanczyn July 2, 1957 2,850,380 Clarke Sept. 2, 1958 

1. A SINGLE-TREATMENT PRECIPITION-HARDENABLE CHROMIUM-NICKEL-MOLYBDENUM ALLOY STEEL ESSENTIALLY CONSISTING OF ABOUT 3.0% TO 14.0% CHROMIUM, 2.5% TO 6.5% NICKEL, 1.0% TO 5.0% COPPER, 4.0% TO 12.0% MOLYBDENUM, UP TO .75% COLUMBIUM, AND THE REMAINDER ESSENTIALLY IRON. 